Full orbit simulation of collisional transport of impurity ions in the MAST spherical tokamak

Abstract

Transport analysis of MAST discharges indicates that collisions are an important loss mechanism in the core of a tight aspect ratio tokamak. In the strongly varying equilibrium fields of MAST many of the assumptions of drift kinetic and neoclassical theory (e.g. small plasma inverse aspect ratio and low ratio of toroidal Larmor radius to poloidal Larmor radius) are not met by all particle species and it becomes appropriate to use full orbit analysis to evaluate heat and particle fluxes. Collisional transport of impurity ions (C 6+ and W 20+ ) has been studied using a full orbit solver, CUEBIT, to integrate the test-particle dynamics. Electromagnetic fields in MAST plasma have been modelled using the cylindrical and toroidal two-fluid codes CUTIE and CENTORI. A detailed study of the scaling of the test-particle diffusivity with collisionality in the equilibrium field reveals deviations from the standard neoclassical theory, in both the Pfirsch-Schlüter and banana regimes, and difficulties in defining a local diffusivity at low collisionalities. The effect of electric and magnetic fluctuations is also briefly addressed. It is found that field fluctuations enhance the non-diffusive nature of transport. The full orbit analysis presented here predicts levels of transport and confinement times for the examined species broadly consistent with the experimental observations.

Heavy impurity confinement in hybrid operation scenario plasmas with a rotating 1/1 continuous mode

“…I. We use these equilibria as the basis for our electric and (9,5) magnetic fields in the guiding-centre orbits. We use the equilibrium to compute the parallel velocity U 1 ,i for the background equilibria.…”

“…One way to model the distribution of impurities is to follow the full 6D gyromotion of the impurity marker distribution (a so-called PIC approach) which is necessary when there are electric and magnetic field variation is of the order of the gyroradius of the particle. When such scalelength variations (including time-varying fluctuations [9]) are not present and the fields are smooth to the scale of the gyroradius, one may follow the guiding-center orbits, which greatly reduces the computational cost. Proper accounting of centrifugal and neoclassical effects leads to the well-known impurity flux distribution [10] in axisymmetry, though little has been done so far using a PIC approach.…”

“…Proper accounting of centrifugal and neoclassical effects leads to the well-known impurity flux distribution [10] in axisymmetry, though little has been done so far using a PIC approach. Modelling attempts in the past have reproduced the strong diffusivity without reproducing the impurity peaking seen in experiments [11][12][13] essentially because the neoclassical transport arising from collisions of the trapped impurity particles with the passing particles background ions was neglected. This is the first area addressed by this manuscript, in which we follow a guiding-center based PIC approach approach with a neoclassical collision operator.…”